Adsorption of UF 6 on Graphene Derivatives: a Computational Study of Conditions for 2D Enrichment
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Adsorption of UF6 on Graphene Derivatives: a Computational Study of Conditions for 2D Enrichment Yang Wei Koh1, Kenneth Westerman2, and Sergei Manzhos3* 1 Bioinformatics Institute, 30 Biopolis Street, #07-10 Matrix, Singapore 138671, Singapore 2 Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, MA, 02155, USA 3 Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576, Singapore *Email: [email protected] ABSTRACT We present a computational density functional theory study of UF6 adsorption on ideal as well as hydrogenated and fluorinated graphene. We show that (i) the isotopic splitting in the vibrational spectrum of UF6 observed in vacuum is largely preserved in the adsorbed molecules. The existence of several adsorption configurations with competing Eads leads to overlaps in the vibrational spectra of isotopomers, but isotopomer-unique modes exist on all three surfaces. (ii) The adsorption energy of UF6 is of the order of 1.2 eV on ideal graphene, 1 eV on graphane, and 0.1 eV on fluorographene, i.e. the adsorption strength is moderate and can be controlled by surface modification. (i) and (ii) mean that it may be possible to cause desorption of a selected isotopomer by laser radiation, leading to isotopic separation between the surface and the gas. INTRODUCTION Isotopomer separation of UF6 based on different absorption properties of 235U and 238U containing molecules is promising economically and is reaching industrial scale (e.g. the SILEX process) [1]. Specifically, separation based on differences in vibrational spectra is enticing, as highly coherent infrared lasers become more and more affordable [2]. Porting this process from the gas phase to a 2D adsorbate system is interesting, as it would make possible higher concentrations of UF6 on an adsorbing surface and a smaller volume to be filled with laser radiation. Separation at a surface could be realized if (i) the isotopic splitting of UF6 vibrational modes is preserved at the surface and (ii) if the adsorption energy could be tuned for selective ad- or desorption of one isotopomer by laser excitation. Indeed, it is known that rates of molecule-surface reactions can be controlled by vibrational excitation of the molecule [3, 4]. If such excitation is isotopomer-selective, accumulation or depletion of selected isotopomers at the surface could be achieved. Here, we present a preliminary density functional theory (DFT) computational study of the conditions (i) and (ii) for UF6 adsorbed at graphene and its hydrogenated and fluorinated analogues. The choice of these materials is guided by the facts that UF6 is known to adsorb on carbonaceous materials [5], these graphene derivatives are feasible to produce [6, 7], and different functionalizations of graphene can be used to tune adsorption energies of molecules [8, 9].
COMPUTATIONAL SETUP Electronic and molecular structures were optimized using DFT [10] with the PBE functional [11] as implemented in
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